How does my correspondent "know" that his neighbors would murder him if they had guns? He doesn't. What he was really saying was that if he had a gun, he might murder his neighbors if he had a bad day, or if they took his parking space, or played their stereos too loud. This is an example of what mental health professionals call projection – unconsciously projecting one's own unacceptable feelings onto other people, so that one doesn't have to own them.3 In some cases, the intolerable feelings are projected not onto a person, but onto an inanimate object, such as a gun,4 so that the projector believes the gun itself will murder him.
Projection is a defense mechanism. Defense mechanisms are unconscious psychological mechanisms that protect us from feelings that we cannot consciously accept.5 They operate without our awareness, so that we don't have to deal consciously with "forbidden" feelings and impulses. Thus, if you asked my e-mail correspondent if he really wanted to murder his neighbors, he would vehemently deny it, and insist that other people want to kill him.
Projection is a particularly insidious defense mechanism, because it not only prevents a person from dealing with his own feelings, it also creates a world where he perceives everyone else as directing his own hostile feelings back at him.6